Question

I’m trying to learn something about initializer_list<>. I learned on c++03 so to me, it is new.

template <typename T>
union Matrix4
{
    struct
    {
        T m00, m01, m02, m03;
        T m10, m11, m12, m13;
        T m20, m21, m22, m23;
        T m30, m31, m32, m33;
    };

    T m[16];
    T mm[4][4];

    Matrix4(std::initializer_list<T> values)
    {
        if (values.size() != 16)
        {
            throw InvalidArgumentException;
        }

        std::copy(values.begin(), values.end(), m);
    }

    //error: Implicit instantiation of "Matrix4<float>" within its own definition.
    static Matrix4<float> Identityf = {
        1.f, 0, 0, 0,
        0, 1.f, 0, 0,
        0, 0, 1.f, 0,
        0, 0, 0, 1.f
    };
}

I get what the error is saying. What I don’t get is why it is saying it. That is why can’t the compiler handle this?

What I would like to accomplish is something like so:

typedef Matrix4<float> Matrix4f;

Matrix4f mat = Matrix4f::Identity;

Answer 1

Initializing the static member IdentifyF needs to call the Matrix4 constructor, which implicitly instantiates the Matrix4<float> specialization, but at the point where it’s instantiated the class template isn’t (fully) defined yet.

§ 14.7.1 [temp.inst]

-7- If an implicit instantiation of a class template specialization is required and the template is declared but not defined, the program is ill-formed.

A class (or a class template) is defined at the closing brace of its class body.

You can make the code work by moving the IdentityF definition out of the class body, to a point where the class template is complete:

template <typename T>
union Matrix4
{
    // ...

    static Matrix4<float> Identityf;
};

template<typename T>
Matrix<float> Matrix4<T>::Identityf = {
    1.f, 0, 0, 0,
    0, 1.f, 0, 0,
    0, 0, 1.f, 0,
    0, 0, 0, 1.f
};

The problem is not related to initializer lists, it would apply to any constructor used at that point (not just a constructor taking an initializer_list)